A known type of wave gear device is a cup-type wave gear device, in which the flexible externally toothed gear has a cup-like shape, and FIG. 4 is a vertical cross-sectional view of a cup-type wave gear device. As shown in the drawing, a wave gear device 1A comprises a circular internally toothed rigid gear 2A, a cup-shaped flexible externally toothed gear 3A coaxially disposed within the internally toothed rigid gear 2A, and an ellipsoidally contoured wave generator 4A, fitted within the flexible externally toothed gear 3A, and adapted for flexing the flexible externally toothed gear 3A into an ellipsoidal shape and for making the flexible externally toothed gear to partially engage with the internally toothed rigid gear 2A.
The cup-shaped flexible externally toothed gear 3A is provided with: a constant-thickness boss 5A constituted by a disc-shaped rigid body; a diaphragm plate 6A extending radially outward from the round outer circumferential surface of the boss 5A, the plate being capable of flexing out of plane; a cylindrical cup body 7A, continuous with the round outer periphery of the diaphragm plate 6A, and extending in the direction of the central axis of the gearing and is capable of flexing radially; and external teeth 8A formed on an external circumferential part of the cylindrical cup body 7A near an open end.
The part of the cylindrical cup body 7A on which the external teeth 8A of the flexible externally toothed gear 3A are formed is ellipsoidally flexed by the wave generator 4A, and the external teeth 8A positioned at the two longitudinal ends of the ellipsoidal curve formed thereby engage with internal teeth 9A of the internally toothed rigid gear 2A. There is a difference of 2n (n being a positive integer; typically, n=1) in the numbers of teeth of the gears 2A, 3A; thus, when the wave generator 4A rotates, the position at which the two gears engage moves in a circumferential direction, generating relative rotation between the gears 2A, 3A corresponding to the difference in the number of teeth. By fixing one gear so as not to rotate and supporting the other gear in a freely rotatable state, the rotatably supported gear rotates at a rotational velocity that is considerably less than that of the wave generator 4A.
The cylindrical cup body 7A of the cup-shaped flexible externally toothed gear 3A is capable of being ellipsoidally flexed by the wave generator 4A, and has a circular cross-section prior to deformation, as shown in FIG. 5(a). After being ellipsoidally flexed by the wave generator 4A, the cylindrical cup body 7A is in a state of gradually expanding outwards from the side thereof nearer the diaphragm plate 6A toward the side thereof nearer an open end 7a when viewed in cross-section along the major axis of the ellipsoidal curve, as seen in FIG. 5(b). Conversely, the cylindrical cup body 7A contracts inward from the side of the diaphragm plate 6A toward the side of the open end, when viewed in cross-section along the minor axis of the ellipsoidal curve, as seen in FIG. 5(c).
In order to enable ellipsoidal flexing of the part of the cylindrical cup body 7A nearer the open end 7a, the cylindrical cup body 7A and the rigid boss 5A are connected by the diaphragm plate 6A. As shown in FIG. 5(b), the diaphragm plate 6A bends back centering on the part where it connects to the boss 5A as indicated by the arrow when viewed in cross-section along the major axis of the ellipsoidal curve. By contrast, when viewed in cross-section along the minor axis, the diaphragm plate 6A is slightly inclined toward the open end 7a, as shown in FIG. 5(c). As the parts of the cylindrical cup body 7A near the open end 7a are repeatedly radially flexed, repeated flexural deformation in a forward/backward direction along the gearing central axis 1a is generated in the diaphragm plate 6A. The diaphragm plate 6A is thereby subjected to torque transmission-induced shear stress and flexural stress arising from the flexural deformation.
The cross-sectional shape of the diaphragm plate 6A is thus engineered so as to allow the combined stress generated by the abovementioned stresses in the diaphragm plate 6A to be reduced and a high level of torque to be transmitted. Patent document 1 (JP-U S61-173851) proposes a cross-sectional shape for relieving the concentration of stress upon the part where the diaphragm plate connects to the boss. In patent document 2 (JP-A H06-017888), the thickness of the portion of the diaphragm plate connecting to the boss is at least three times the minimum thickness of the diaphragm plate, and the thickness gradually decreases in a radial outward direction away from the portion connecting to the boss, thereby preventing excessive stress being concentrated upon the cup-shaped flexible externally toothed gear, which has a short axial length. Patent document 3 (JP-A 2006-057684) proposes a method of defining the thickness of the various parts of a diaphragm plate in a cup-type wave gear device having a high gear ratio so as to increase load capacity.